Naval aircraft and related equipment operate in an environment which is unique in that the load carrying surfaces such as bearings, splines, gears and alike in addition to experiencing wear under normal operating conditions, must also function in a highly corrosive environment. This requirement places substantial burden on the lubricating additives that must function as a corrosion inhibitor and as an extreme pressure agent under severe environmental conditions and at times at relatively high temperatures. Problems relating to corrosion and wear have in the past been treated as separate problems whereas in reality corrosion and wear resistance are primarily surface sensitive requirements. Accordingly, there is substantial interest in lubricating additives which exhibit corrosion resistance and at the same time improve the wear resistance under extreme environmental and operating conditions.
Presently, there is no single lubricating additive which functions both as an anti-wear (lubricating agent) and a corrosion-inhibiting additive. Some of the known lubricants including the solid lubricants such as molybdenum disulfide is known to hydrolyse forming acidic components which readily attack metal causing corrosion. Similarly graphite, although known as a dry lubricant, is capable of forming a galvanic cell with bearing metals and acts as a cathode thereby resulting in corrosion. The lubricating additives of this invention, however, were found not only to inhibit corrosion but also to have the unique capability of performing as an anti-wear agent in various grease compositions. The additives of this invention are very useful for military purposes, and can be used in lubricants in high performance engines and particularly for aircraft which have sophisticated bearings, gears and other working parts. These engines are required to perform at substantially higher loads and speeds, and at higher temperatures thereby reducing the life of the lubricants.
A substantial increase in the life of a bearing by improving the lubricant, for example, will not only reduce the high maintenance cost due to down time, which is critical in both commercial and military aviation, but is useful also in the auto industry which is continually trying to improve petroleum products, particularly for its super-charged engines which require hihger operating temperatures and increased loads. These high temperatures and loads require the bearings, for example, to operate under substantially more demanding conditions. Therefore, it was unexpected to find substantial improvement by using the Schiff base compounds of this invention as additives in greases in machinery aboard ship, submarines and particularly in the aircraft industry.
More specifically, studies have shown that there is a unique relationship between wear and corrosion and that the enhancement of passivity or build-up of corrosion resistance also significantly reduces wear. Presently, solid lubricants such as molybdenum disulfide, graphite and alike are primarily used as lubricating additives at elevated temperatures under extreme loads. However, these dry lubricants while improving the load and extreme pressure qualities of the lubricant do not have any intrinsic corrosion-inhibiting characteristics. As indicated herein, molybenum disulfide hydrolyzes to form acidic components which readily attack the metal causing corrosion. Similarly, graphite is an electro-chemically noble material and is therefore known to form galvanic cells with bearing metals in the presence of moisture or any ionic medium causing corrosion. Other known compounds such as chromates, sulfonates, molybdates, nitrites and alike are known to inhibit corrosion only under certain conditions. Moreover, while some of these compounds improve the corrosion protection of a particular lubricant, these same compounds do not, however, improve the wear characteristics of the lubricant under extreme pressure and at higher temperatures.